In recent years, transformers with super high voltages (higher than 1 million volts) which are of large size are being developed in accordance with an increased demand for electric power.
Accordingly, in order to obtain an improvement of insulating reliability, the required properties for electric insulating oils are becoming more severe. Particularly, in insulating oils for super high voltage transformers, an increment of dielectric loss tangent (tan .delta.) accompanied by deterioration of thermal stability is becoming a serious problem. The heat generated in a dielectric is increased on account of the increased tan .delta., which results in heating generation of the transformers.
Regarding the increased tan .delta., various causes are considered, and yet, sufficient elucidation has not been brought about. There is a report that presence of copper exerts an influence. Under the circumferences, an effort has been made to suppress the increment of tan .delta. by adding a small amount of 1,2,3-benzotriazole (BTA), which reacts quantitatively with copper, to ordinary mineral base oil. But, it is difficult to prevent the increment of tan .delta. completely, and moreover, an introduction of an improved technique has been hoped for.